I noticed while looking through the code that you can point your blog to a different users table just by defining CUSTOM_USER_TABLE in your wp-config.php file. This works well if you want all your blogs to have the same users and permissions, but only for that scenario. My setup is that I have 8 different WP blogs setup on my site, one for each person. I would love it if the blog permissions could get split out of the users table in a future version of WP. This would allow maintaining a single user table and have different permissions for each blog. If this were combined with the MU work I think it would be a great setup for small communities of bloggers.
Is there any other interest in this?